1. Field of the Invention
The present invention relates to a method and an apparatus for detecting dynamic change in ultrasonic wave or the like propagating through a medium. Further, the present invention relates to an ultrasonic diagnostic apparatus having such a dynamic change detecting apparatus.
2. Description of a Related Art
In an ultrasonic diagnostic apparatus for a so-called ultrasonic echo observation or the like, it is the general practice to use piezoelectric materials typically represented by PZT (Pb (read) zirconate titanate) for an ultrasonic sensor portion (probe).
FIGS. 12A and 12B schematically show the structure of a conventional probe. FIG. 12A is a whole perspective view of the probe, and FIG. 12B is an enlarged perspective view of array vibrator included in the probe.
As shown in FIG. 12A, the probe 301 has a thin box shape as a whole, and has a slender rectangular probing surface 302. The probing surface 302 is brought into contact with a human body and an ultrasonic wave is transmitted so as to receive an ultrasonic echo reflected from the depths of the body. A cable 307, which transmits a drive signal for transmitting an ultrasonic wave and a detection signal of the ultrasonic wave, is connected to the upper side of the probe 301.
A comb-shaped array vibrator 303 serving as both a transmitter and a receiver of ultrasonic wave is housed in the probing surface 302. As shown in FIG. 12B, the array vibrator 303 is provided a number of slits 306 (having a width of, for example, 0.1 mm) in a thin strip-shaped PZT sheet (having a thickness of, for example, 0.2 to 0.3 mm) so as to form a number of (for example, 256) comb-teeth-shaped individual vibrator 305 (having, for example, a width of 0.2 mm and a length of 20 mm).
An electrode is formed in each individual vibrator 305, and a signal line is connected thereto. An acoustic lens layer or an acoustic matching layer made of resin material such as rubber is attached to the surface side (lower side in the drawing) of the array vibrator 303, and a backing material is attached to the back side. The acoustic lens layer converges the transmitted ultrasonic waves effectively. The acoustic matching layer improves the transmission efficiency of ultrasonic waves. The backing material has a function of holding the vibrator and causes vibration of the vibrator to be finished earlier.
Such ultrasonic probe and ultrasonic diagnostic apparatus are described in detail in xe2x80x9cUltrasonic Observation Method and Diagnostic Methodxe2x80x9d, Toyo Publishing Co., or xe2x80x9cFundamental Ultrasonic Medicinexe2x80x9d, Ishiyaku Publishing Co.
In the field of ultrasonic diagnostic, it is desired to collect three-dimensional data in order to obtain more detailed information about the interior of an object""s body. In order to comply with such a demand, it is required to make ultrasonic detecting elements (ultrasonic sensors) into a two-dimensional array. In the aforementioned PZT, however, it is difficult to fine the devices down and integrate them in the present conditions for the following reasons. That is, processing technology of PZT materials (ceramics) is almost on a limit level, and further fining down leads to an extreme decrease in processing yield. Moreover, if the number of wires increases, electrical impedance of the element and crosstalk between the elements (individual vibrators) would increase. It is therefore considered difficult to realize a two-dimensional array probe using PZT at the present level of the art.
On the other hand, Japanese patent application publication JP-A-10-501893 discloses an ultrasonic detecting apparatus including an array of vertical cavity surface emission laser (VCSEL) excited electrically (pumping). A cavity length of each laser is modulated by the acoustic field propagated from an object. As a result, the laser beam obtained thereby is frequency modulated by the acoustic field. The modulated laser beam is converted into amplitude modulation signal by a detector head, and thereafter, detected by a CCD array. Then, information of the signal is transmitted electrically to the signal processing assembly and processed. It is stated that this ultrasonic apparatus can achieve high level detection of frequency bandwidth, high resolving power of space and simplification of electric wiring.
Further, a paper entitled xe2x80x9cHigh Frequency Ultrasound Imaging Using an Active Photodetectorxe2x80x9d by James D. Hamilton et al. appears in IEEE TRANSACTIONS ON ULTRASCONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 45, NO. 3, MAY 1998. This paper discloses an ultrasonic detecting apparatus including laser and optical modulator having a waveguide made of neodymium doped glass.
However, the detection system using change in a length of the laser resonator due to the ultrasonic wave have no practicality without compensation for environmental changes such as temperature change because such a detection system has high sensitivity for displacement. In the case of arraying the sensors, since variance will be inevitably generated in oscillation frequencies at respective laser element, it would be difficult to put the sensors to practical use as an array, unless a measuring method which is not affected by the variance in oscillation frequencies of laser elements is used.
The present invention has been accomplished in view of these problems. A first object of the present invention is to provide a dynamic change detecting method and apparatus for detecting dynamic change stably by canceling influence of environmental change or an individual difference between a plurality of laser elements. A second object of the present invention is to provide an ultrasonic diagnostic apparatus using such a dynamic change detecting apparatus and appropriate for collection of three-dimensional data.
In order to solve the aforementioned problems, a dynamic change detecting method according to the present invention comprises steps of: (a) emitting a laser beam while causing frequency modulation to the laser beam to be generated in accordance with change in a size of a laser resonator by propagating the dynamic change to a total reflection mirror included in the laser resonator to cause dynamic displacement to the total reflection mirror; (b) separating the laser beam into a plurality of split-beams and guiding the plurality of split-beams to a plurality of optical paths having mutually different optical path lengths, respectively; (c) causing frequency shift in at least one of the plurality of split-beams; (d) combining the plurality of split-beams with each other to obtain interference light, and detecting the interference light to obtain an intensity signal corresponding to intensity of the interference light; (e) demodulating the intensity signal to generate a demodulated signal; and (f) obtaining a signal corresponding to the dynamic change on the basis of the demodulated signal.
Moreover, a dynamic change detecting apparatus according to the present invention comprises a laser including a laser resonator having a total reflection mirror where a dynamic perturbation is generated by propagation of dynamic change, the laser emitting a laser beam while causing frequency modulation to the laser beam to be generated in accordance with change in a size of the laser resonator; first means for separating the laser beam emitted from the laser into a plurality of split-beams and guiding the plurality of split-beams to a plurality of optical paths having mutually different optical path lengths, respectively; second means for causing frequency shift in at least one of the plurality of split-beams; third means for combining the plurality of split-beams with each other to obtain interference light; a photodetector for detecting the interference light to obtain an intensity signal corresponding to intensity of the interference light; fourth means for demodulating the intensity signal to generate a demodulated signal; and fifth means for obtaining a signal corresponding to the dynamic change on the basis of the demodulated signal.
Further, an ultrasonic diagnostic apparatus according to the present invention comprises transmitting means for transmitting an ultrasonic wave; receiving means for receiving an ultrasonic echo to convert the ultrasonic echo into an electric signal, the receiving means comprising a laser, including a laser resonator having a total reflection mirror where a dynamic perturbation is generated by propagation of dynamic change, for emitting a laser beam while causing frequency modulation to the laser beam to be generated in accordance with change in a size of the laser resonator, means for separating the laser beam emitted from the laser into a plurality of split-beams and guiding the plurality of split-beams to a plurality of optical paths having mutually different optical path lengths respectively, means for causing frequency shift in at least one of the plurality of split-beams, means for combining the plurality of split-beams with each other to obtain interference light; a photodetector for detecting the interference light to obtaining an intensity signal corresponding to intensity of the interference light, means for demodulating the intensity signal to generate a demodulated signal, and means 45 for obtaining a signal corresponding to the dynamic change on the basis of the demodulated signal; and image processing and displaying means for image processing of the signal corresponding to the dynamic change and displaying an image on the basis of the signal.
According to the present invention, the separated laser beams pass through the optical paths having different optical path lengths respectively and the frequency shift is generated in at least one of the separated laser beams, and thereafter, the separated laser beams are combined with each other so as to cause interfere light (so-called heterodyne interference) even if the detection environment, for instance, the temperature is changed in the laser resonator. Therefore, intensity of the interfere light is hardly affected by the temperature change and the dynamic change of an object can be detected stably. In addition, in the case where the laser resonator is constructed as an array, the dynamic change of an object can be detected stably even if oscillation frequencies of respective resonator units have dispersion.